Stents are typically endovascular prostheses that are used for the therapy applied to stenoses. They essentially comprise a support structure by which the wall of a vessel, such as, for example, an artery, is braced so as to ensure sufficient flow through the vessel. In addition, an aneurysm can be bridged by the stent. During implantation, the stent is inserted in a compressed state into the vessel and then expanded at the site to be treated. Expansion of the stent is typically effected by means of a balloon catheter that has been previously inserted into the interior of the stent and also functions to position the stent within the vessel. As a result of the expansion of the stent, the walls of the support structure are pressed against the vascular wall, thereby effecting an adequate sectional area of flow for the vessel. In order to ensure that the sectional area of flow is not reduced too much by the stent itself, stents generally have very narrow wall thicknesses in the region of the support structure. These narrow wall thicknesses must, however, ensure that the expanded shape of the stent is preserved despite a pressure applied by the vessel and acting radially on the stent. In addition to radial strength, the requirement also to be met by the stent is to have sufficient flexural stiffness to enable the stent to adapt as well as possible to the partially bent shape and the movements of the vascular section in which it is implanted.
Typically, the support structure of the stent here is essentially lattice-shaped, although this lattice can have a wide variety of designs. The lattice structure is typically generated by a laser cutting process targeted at a lateral cylindrical surface of a tube.
The struts forming the lattice structure thus have an essentially rectangular cross section, as shown in FIG. 2 which illustrates the prior art. The disadvantageous aspect of the embodiment of the strut 200 shown in FIG. 2 is that the blood flow 211 passing over strut 200 tends to form turbulences 213, with the result that a turbulent flow is created in the region of strut 200 instead of the desired laminar flow. Turbulences 213 result in unwanted deposits along with the possible consequential effects of neointimal hyperplasia and arthrosclerosis 212. These deposits can develop further into a symptomatic restenosis. This effect is found particularly in the case of proximally-located struts and decreases in the distal direction.
Various measures are known involving treating the struts after the laser-cutting procedure. The struts are treated, for example, by electropolishing. The rounding of edges produced thereby is so small, however, that it does not produce any significant change in the flow from turbulent into laminar. In addition, electropolishing is used to treat the entire support structure, with the result that it is not possible to perform a selective treatment of the proximal end of the stent. Furthermore, the slight removal of material here essentially is effected symmetrically over the strut, that is, on the mural edges, the edges facing the vascular wall, and on the luminal edges facing the vascular lumen—all of which does not counteract the formation of turbulences.
An ellipsoid cross-sectional surface for the strut is known from EP 0 824 903 A1 which differs from the cross section of a strut as described and illustrated in FIG. 2. A cross-sectional shape is shown in FIG. 2 of this publication in which both the luminal as well as the mural surfaces are curved around the longitudinal axis of the strut. It is the curvature of the mural surface in particular that is disadvantageous in that a strut curved in this way penetrates more deeply into the vascular wall during dilatation, thereby damaging this wall to a greater extent. The result is an increase in neointimal proliferation which in turn promotes the formation of deposits. The luminal surface of the strut is curved symmetrically. It is well known that symmetrical curvatures have an adverse effect on fluid flows in so far as they are unable to produce a laminar flow. This means that turbulences of the blood flow passing over the luminal surface occur even in this ellipsoid embodiment shown in EP 0 824 903 A1, which turbulences in turn also result in the formation of deposits. Another disadvantage of the strut cross sections referenced in this document is the reduction in its axial section modulus which in particular has a disadvantageous effect on the strength of the stent in response to increased bending stress. In addition, the radial strength of the stent is lowered by the decreased wall thickness of the support structure.
Particularly in the case of novel stents composed of magnesium alloys, relatively large wall thicknesses are required for reasons of strength, with the result that the cross-sectional shapes of the struts disclosed in EP 0 824 903 A1 are not usable for such stents without unduly reducing the radial strength of the stent. In order to fabricate stents from magnesium alloys that have the requisite radial strength and using these types of web cross sections as known in the referenced prior art, the struts would have to have significantly larger dimensions in cross section. This is disadvantageous, however, because the sectional area of flow of the blood vessel is reduced thereby, which effect results in turbulences and increased formation of deposits.
US 2008/0082162 A1 discloses various strut cross sections, although these have a coating on their mural surface. Curvatures of luminal strut surfaces are of symmetrical form, as a result of which once again the referenced unwanted deposits appear on the strut when inserted in the blood stream.
The fundamental problem to be solved by this invention is to provide a stent, as well as a method of fabricating the stent, wherein the objective is to design the stent such that vascular constriction is minimized while at the same time sufficient flexural and radial stiffness of the stent is provided.